The present invention relates in general to non-volatile memory circuits and, more particularly, to measuring the negative threshold voltage of a non-volatile memory cell.
Electrically erasable programmable read only memories (EEPROMs) are found in many applications where it is necessary to use a non-volatile memory. One such application is in the field of portable data carriers (PDC), otherwise known as smart cards. A PDC is generally made of plastic, about the size of a conventional credit card, and includes one or more semiconductor die embedded in the PDC. The semiconductor die(s) include a microprocessor, memory, and various input and output (I/O) circuitry. While a conventional credit card with a magnetic strip typically stores a few hundred bits of data, the PDC with its expanded memory can store 8K or more 8-bit bytes of data.
The additional storage capacity of the PDC vastly expands its useful applications. For example, the PDC can be used to store the user's medical history. The user presents the PDC to a health care provider who, through a PDC reader, extracts the patient's medical history, including personal data, primary care physician, health insurance, allergies, medication, past procedures, blood type, religious preference, organ donor, etc. Other applications for PDCs include banking services, identification for nationality and passport, and transportation transactions such as ticket and fare collection. For example, the PDC can be programmed to hold a monetary value. When making a purchase, the user inserts the PDC into the PDC reader and the purchase amount is automatically deducted from the stored monetary value. PDCs are applicable virtually anywhere the user needs to convey or exchange data or information.
The PDC is available to operate in contact and contactless modes. In contact mode, the PDC is inserted into the PDC reader. The PDC reader comes in direct electrical contact with terminal pads on the PDC to supply operating power and to read and write data. In contactless mode, the PDC uses radio frequency (RF) transmission circuitry. The contactless PDC is placed in the vicinity of the PDC reader and the information exchange occurs over the RF link.
The PDC generally does not contain a local power source such as a battery. The PDC receives operating power at the beginning of each transaction by the direct electrical contact, or via the RF link. The memory area on the PDC is divided between random access memory (RAM), read only memory (ROM), and EEPROM. The RAM is volatile memory and maintains temporary data used only during the time that power is supplied by the PDC reader. The ROM and EEPROM are non-volatile memory and, although can only be accessed during the time that power is supplied by the PDC reader, maintain their contents even during times of zero operating power.
The EEPROM array is arranged in a matrix of memory cells. Each memory cell has a floating gate transistor that stores a logic one or a logic zero. A logic one is stored as a positive charge on the floating gate, and a logic zero is stored as a negative charge on the floating gate. A floating gate transistor with a negatively charged floating gate has a positive threshold voltage (VT) related to the stored charge. A floating gate transistor with a positively charged floating gate has a negative threshold voltage related to the stored charge. A fully charged floating gate yields a VT of about .+-.5 volts.
Once the EEPROM cell is written, the stored charge decays over time because of leakage. The stored charge typically has a life expectancy of about 10 years before the VT drops below a minimum value of about .+-.2 volts and the cell contents become unpredictable.
One way of determining the life expectancy of an EEPROM cell is to measure its VT during the manufacturing process. If the measured VT is greater than a specified value, e.g. 5 volts, then the stored charge is expected to last the projected lifetime. If the measured VT is less than the specified value, then the stored charge is unlikely to last the projected lifetime. If the VT begins at a level lower than the specified value, then as the stored charge decays over time, the VT will become less than the minimum acceptable value before the end of its projected life expectancy. When EEPROM cells with unacceptably low VTs are detected, the manufacturing process is evaluated for problems.
There are known techniques to measure a positive VT. For example, a variable positive voltage can be applied to the floating gate and increased until the transistor turns on. The point that the floating gate transistor turns on is the positive VT. However, the negative VT of the floating gate transistor cannot be measured in the same manner.
Hence, a need exists to measure the negative VT of a non-volatile memory cell.